Astronomy & Astrophysics publishes the work of
researchers from the University of Vienna, who have found a river of
stars, a stellar stream in astronomical parlance, covering most of the
southern sky. The stream is relatively nearby and contains at least
4000 stars that have been moving together in space since they formed,
about 1 billion years ago. Due to its proximity to Earth, this stream
is a perfect workbench on which to test the disruption of clusters,
measure the gravitational field of the Milky Way, and learn about
coeval extrasolar planet populations with upcoming planet-finding
missions. For their search, the authors used data from the ESA Gaia
satellite.

Our own host galaxy, the Milky Way, is home to star clusters of
variable sizes and ages. We find many baby clusters within molecular
clouds, fewer middle-age and old age clusters in the Galactic disk, and
even fewer massive, old globular clusters in the halo. These clusters,
regardless of their origin and age, are all subject to tidal forces
along their orbits in the Galaxy. Given enough time, the Milky Way
gravitational forces relentlessly pull them apart, dispersing their
stars into the collection of stars we know as the Milky Way.

“Most star clusters in the Galactic disk disperse rapidly after their
birth as they do not contain enough stars to create a deep
gravitational potential well, or in other words, they do not have
enough glue to keep them together. Even in the immediate solar
neighborhood, there are, however, a few clusters with sufficient
stellar mass to remain bound for several hundred million years. So, in
principle, similar, large, stream-like remnants of clusters or
associations should also be part of the Milky Way disk.“ says Stefan
Meingast, lead author of the paper published in Astronomy &
Astrophysics.

Thanks to the precision of the Gaia measurements, the authors
could measure the 3D motion of stars in space. When carefully looking
at the distribution of nearby stars moving together, one particular
group of stars, as yet unknown and unstudied, immediately caught the
eye of the researchers. It was a group of stars that showed precisely
the expected characteristics of a cluster of stars born together but
being pulled apart by the gravitational
field of the Milky Way.

"Identifying nearby disk streams is like looking for the proverbial
needle in a haystack. Astronomers have been looking at, and through,
this new stream for a long time, as it covers most of the night sky,
but only now realize it is there, and it is huge, and shockingly close
to the Sun” says João Alves, second author of the paper. "Finding
things close to home is very useful, it means they are not too faint
nor too blurred for further detailed exploration, as astronomers
dream."

Due to sensitivity limitations of the Gaia observations, their
selection only contained about 200 sources. An extrapolation beyond
these limits suggests the stream should have at least 4000 stars,
thereby making the structure more massive than most know clusters in
the immediate solar neighborhood. The authors also determined the
stream’s age to be around one billion years. As such, it already has
completed four full orbits around the Galaxy, enough time to develop
the stream-like structure as a consequence of gravitational interaction
with the Milky Way disk.

“As soon as we investigated this particular group of stars in more
detail, we knew that we had found what we were looking for: A coeval,
stream-like structure, stretching for hundreds of parsecs across a
third of the entire sky.” Says Verena Fürnkranz, co-author and Masters
student at the University of Vienna. "It was so thrilling to be part of
a new discovery" she adds.

This newly discovered nearby system can be used as a valuable gravity
probe to measure the mass of the Galaxy. With follow-up work, this
stream can tell us how galaxies get their stars, test the gravitational
field of the Milky Way, and, because of its proximity, become a
wonderful target for planet-finding missions. The authors hope to
unravel even more such structures in the future with the help of the
rich Gaia database.

Fig. 1. Night sky centered on the
south Galactic pole in a so-called stereographic projection.
In this special projection, the Milky Way curves around the entire image in an
arc. The stars in the stream are displayed in red and cover almost the
entire southern Galactic hemisphere, thereby crossing many well-known
constellations. Background image: Gaia DR2 skymap.